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Thread: Twist rates

  1. #1

    Twist rates

    Ok, so I was flipping through some pages on Midway UK, looking for a target and varmint bullet (note, fox and similar, not deer) for this spring. I figured I might give the Bergers a shot (excuse the pun), but was somewhat surprised by their twist recommendations. They note that the 95g, 100g, and 105g bullets are recommended for 1:8/1:9 twists, the 90g at 1:10, and the lower such as 70g for 1:13/1:14 twists.

    I have calculated that my ideal bullet is indeed 90g for my 1:10 twist, but was very surprised to read that such slow, and fast twist rates are recommended for bullets just marginally on the +/- side of the 90g mark...or perhaps they are referring to what is needed for 'ultimate' accuracy at longer range target shooting, and in reality, either side of the 90g scale would should pretty good within say 100-300 yds?

  2. #2
    PKL

    Twist rate is important. The late moe243 watched me put 105 Amaxs sideways through a target out of my 243 when it had a 1:10 twist barrel. It now has a 1:8 twist barrel (as per my gallery) and shoots the Amaxs quite well.

    Regards JCS

  3. #3
    You're forgetting the big factor about twist rates. It is not necessarily governed by bullet weight, but rather by bullet length. Determining twist rate purely by weight is senseless.~Muir

  4. #4
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    Quote Originally Posted by Muir View Post
    You're forgetting the big factor about twist rates. It is not necessarily governed by bullet weight, but rather by bullet length. Determining twist rate purely by weight is senseless.~Muir
    +1

    The Berger VLD (Very Low Drag) bullets in .243 are very long, that is why there is such a variation. If you look at other manufacturers you will see that they do not have such tight twist rates. You will also find that the VLD's do not fit into most rifle magazines either due to their length.

    I have been using bergers for a few months now and the consistency is very impressive. The 80grain Bergers will give me 0.5" groups at 200yds all day.

  5. #5
    Gyroscopic stability is a function of the rotational velocity of the bullet relative to the length of the bullet. Weight is not a factor other than: When the MATERIALS are the same - meaning density is the same - then WITHIN A CALIBER it MUST follow that heavier bullets are LONGER. Assuming that hunting bullets are fundamentally the "same" density, weight is an INDICATOR of LENGTH, which in turn determines gyroscopic stability. The actual characteristic that is only rarely noted when discussing gyroscopic stability is revolutions per unit time - usually minutes - or RPMs.

    It doesn't matter what the twist rate is, if the velocity is not sufficient to obtain the minimum RPM value for gyroscopic stability for a bullet of a given length, the bullet will NOT be gyroscopically stable. In other words, twist rate and length are not enough information to determine if a bullet will be gyroscopically stable. You also MUST consider muzzle velocity which, WITH twist rate, determine the rotational velocity (RPMs). That number (RPM) ALONG WITH the length of the bullet, determines if the bullet will be gyroscopically stable. (There are other smaller factors like where the center of mass is relative to the center of form, but that ratio is fairly uniform in most hunting bullets so it can be considered a constant and ignored. Just like density.) As a general rule, for most hunting bullets, if you can get the bullet doing 180,000 RPMs or more, it is likely to be gyroscopically stable over 'typical' hunting ranges.

    To calculate RPMs you need the muzzle velocity and the twist rate and you need to pay attention to what your science teachers told you about "Units".

    Muzzle Velocity = FOOT per SECOND
    Twist Rate = REVOLUTIONS per FOOT

    Length units cancel and you end up with revolutions per unit time.

    If the MV is 3000 feet per SECOND then it is 180,000 feet per MINUTE.
    If the Twist Rate is 1:12 then there is 1 revolution (turn) in 12 inches (one FOOT).
    The "foot" units cancel, and the revolutions per minute (RPMs) are 180,000

    If you change the Twist rate to 1:9, that's 1.333 revolutions in one foot of travel. If the bullet travels 180,000 feet in one minute, then the RPMs are 1.333 x 180,000 = 240,000.

    If you get long 'pencils' like 160-grain 6.5mm (.264") bullets, in slow twist rates, AND push them at low muzzle velocities, they will NOT gyroscopically stabilize. If you increase the twist rate (requires a change in barrel), and keep the velocity the same, you increase the RPMs and the stability. If you increase the velocity (change your load or bullet length (weight)) and keep the twist rate the same, you increase the stability. One of the reasons long 'pencils' like 160-grain 6.5mm bullets seem to stabilize "above their pay grade" is that because they are almost always round-nosed, their center of form and center of mass are almost coincident. That helps stability. Conversely, the same WEIGHT bullet in the SAME caliber, but with a LONG, 12-radius, secant ogive, will not be gyroscopically stable in the same twist barrel at the same muzzle velocity.

    It's this simple: If the bullet you want to shoot is not gyroscopically stable, you have three choices:
    1) Change the bullet (shorter)
    2) Change the barrel (increase twist rate)
    3) Change the velocity (increase).

    Regards,
    Paul

    PS - This is precisely why many of us "old guys" don't like the "new" monometal bullets. Lead is VERY dense relative to all the "socially correct" metals. Therefore, you can make a SHORT but HEAVY lead bullet. When you use "monometals", that same-weight bullet, in the same caliber, MUST be longer. It WILL therefore REQUIRE a higher velocity or a faster twist rate to gyroscopically stabilize it.

    Paul
    Last edited by gitano; 27-01-2011 at 17:03.

  6. #6
    Quote Originally Posted by gitano View Post
    It's this simple: If the bullet you want to shoot is not gyroscopically stable, you have three choices:
    1) Change the bullet (shorter)
    2) Change the barrel (increase twist rate)
    3) Change the velocity (increase).
    Paul. Thanks, I changed the barrel. This gave me access to a range of 6mm bullets weighing over 100gr. Rgds JCS

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